In an unusually pointed letter, the staff of the Maryland Public Service Commission pressed Bell Atlantic this week to set a definitive end date for its market trial of high-speed digital home telephone service.

The letter, signed by PSC Telecommunications Division chief Michael Starkey, raises questions about whether Bell Atlantic's residential trial of the service -- known as ISDN -- is in fact a trial or an attempt to implement its proposed price structure without a public hearing.

Shannon Fioravanti, a Bell Atlantic spokeswoman, denied that the trial was anything but genuine. She said the company would be "glad to have the PSC's input" in pricing the service.

Bell Atlantic is now offering ISDN service, which lets users transmit data at about five times the speed of the fastest conventional modems, in a trial that began in Maryland this summer.

The ISDN, or integrated services digital network, has aroused considerable interest among computer users, who see it as a way to tap the potential power of the Internet. But phone companies have moved cautiously in using the service.

In September, Bell Atlantic sought approval for a permanent tariff, but withdrew the proposal in October after computer users protested the lack of a flat-rate option. At the time, Bell Atlantic said it would continue the trial for an indefinite period.

Mr. Starkey's letter pressed Bell Atlantic to set an exact date for the expiration of the trial. He also urged the company to test a variety of pricing options, rather than simply setting the test price at the level of the withdrawn tariff. Mr. Starkey suggested that the company should propose such rates by Jan. 1, so the trial period can be completed in early 1996.

His letter expressed surprise that Bell Atlantic-Maryland has begun charging customers for installing ISDN during the trial period.

"This is a change from [Bell Atlantic-Maryland's] original trial operations and, in combination with extending the service to consumers statewide at [the company's] original proposed rates, begs the question as to whether ISDN is indeed a trial service in any real sense," the letter stated.

News of the PSC letter was welcomed by consumer advocates. "We're thrilled," said James Love, director of the Washington-based Consumer Project on Technology, which has been fighting Bell Atlantic's ISDN plans throughout the company's service region.

Bell Atlantic had proposed a metered rate for ISDN service of 2 cents a minute during peak-use times and a penny a minute during other periods. The company initially rejected the idea of a flat rate, arguing that some users would keep their lines active around the clock and would strain the network's capacity.